1. Field of the Invention
The present invention relates to a filter in a millimeter wave band, and more particularly to a millimeter wave band filter to which the technology of a non-radiative dielectric waveguide (xe2x80x9cNRD guidexe2x80x9d) is applied.
2. Description of Prior Art
An NRD guide circuit has attracted attention as a transmission line for a micro wave band, particularly a millimeter wave band above 30 GHz, due to its small transmission loss in comparison with a microstrip line and due to its easiness in manufacturing the transmission line in comparison with prior waveguides.
The structure of a general prior NRD guide circuit is illustrated in FIG. 1. The NRD guide circuit has a structure that a dielectric line 10 through which an electromagnetic wave is transmitted is sandwiched between two parallel conductive plates 12a, 12b made from conductive metal. A space h of the two parallel plates 12a, 12b is less than half a free space wavelength of a using frequency. Accordingly, the electromagnetic wave is blocked in places other than the dielectric line 10 and its radiation is restricted, so that the NRD guide circuit can transmit the electromagnetic wave along the dielectric line 10 at a small loss. Paying attention to such transmission superiority of the NRD guide circuit, there have been proposed NRD guide filters of the 35 GHz and 50 GHz bands.
FIGS. 2 and 3 are perspective views illustrating the structure of a prior air gap coupled filter using an NRD guide. The prior air gap coupled filter has a structure that multi-staged dielectric blocks are sandwiched between the parallel conductive plates 12a, 12b. One dielectric line is cut into plural dielectric blocks with proper lengths. The dielectric blocks 14axcx9c14e are aligned in a line, with maintaining certain gaps therebetween, in the direction to which a signal proceeds and is air gap coupled with dielectric lines 10a, 10b on input and output sides, respectively. Each of the dielectric blocks operates as a dielectric resonator at each stage of the filter. The number of the dielectric resonator blocks is proportional to a filtering order number of the filter. The air gap coupled filter shown in FIG. 2 is the fifth order filter because it has five dielectric resonator blocks 14axcx9c14e. 
The typical raw material for the dielectric line of the NRD guide which is applicable to millimeter waves is teflon. Teflon has an advantage that transmission loss is small whereas it has such disadvantages arising from its material characteristic that its processing is difficult due to its weakness and that its assembly is difficult because it does not easily adhere to other materials like metal. These disadvantages are the reason why the NRD guide has not been commercially used since the first introduction by Professor Yoneyama in the early 1980""s.
Since the using frequency is as high as the millimeter wave band, a wavelength of the electromagnetic wave transmitted along the dielectric resonator blocks in the waveguide, i.e., within the parallel conductive plates, is very short. The characteristic of the filter, in this case, is sensitively changed in accordance with the physical dimensions of structural bodies and fixtures for setting the resonator. Thus, it is necessary not only that a length of each of the dielectric resonator blocks 14axcx9c14e should be so accurately calculated as to resonate at a certain frequency within a passing band, but also that each of the dielectric resonator blocks should be made as precisely as a predetermined length so as to obtain a wanted characteristic of the filter.
Further, each of the multi-staged dielectric resonator blocks 14axcx9c14e should be spaced a proper gap apart from its adjacent dielectric resonator blocks. This gap should be determined to obtain an optimal impedance matching between the two adjacent resonator blocks. That is, in order to obtain a good characteristic of a designed filter, there should be a precision of several microns not only in the length of each of the dielectric resonator blocks 14axcx9c14e but also in the distance between the resonators.
However, in manufacturing the prior air gap coupled filter using the NRD guide, it is difficult to make the dielectric resonator blocks 14axcx9c14e have such a precision. And also, with maintaining the precision of several microns, it is difficult to align the dielectric resonator blocks 14axcx9c14e having different lengths in a straight line in the direction that a wave proceeds. In doing so, a lot of time and labor are required. Due to these reasons, the prior air gap coupled filter is a disadvantageous structure in terms of making, assembly and production, and is not suitable for a commercial model which is applicable to a high frequency in the millimeter wave band.
In order to improve the above problems, an object of the present invention is to provide a metal post filter assembly, using an NRD guide, which is designed for an easy making and a good productivity resulting from a convenient and accurate assembly and is capable of stably having filter characteristics to a wanted degree.
To accomplish the above object of the present invention, there is provided a metal window filter assembly using an NRD guide, comprising a filter housing which includes parallel conductive plates facing each other and a filter, disposed between the parallel conductive plates, for filtering a certain frequency band of an electromagnetic wave traveling therethrough, the filter including a plurality of polygonal metal windows and a single body type dielectric line made from a non-radiative dielectric, the dielectric line being formed with a plurality of polygonal inserting grooves which are spaced by the predetermined distance on first and/or second surfaces of the dielectric line making contact with the parallel conductive plates, and the metal windows being inserted in the inserting grooves one to one to form multi-staged dielectric resonators cascaded as a single body.
The metal windows provide discontinuous surfaces which radiate with respect to the electromagnetic wave. The multi-staged dielectric resonators have an impedance coupling relationship by the metal windows"" positions and sizes and a reflection amount of, and a transmission amount of, the electromagnetic wave transmitted by the impedance coupling relationship is properly determined. As a result, the filter becomes to provide a filtering function selectively passing the certain frequency band.
It is preferable that an impedance of the multi-staged dielectric resonators is largest in a middle stage and becomes gradually and symmetrically smaller to both end stages.
For a phase compensation of the electromagnetic wave, it is preferable that a length of each stage of the dielectric resonators divided by the metal windows becomes gradually shorter from a middle stage to both end stages.
According to a preferred example of the filter, each of the inserting grooves has an identical width whereas depths of the inserting grooves become gradually and symmetrically deeper to a middle stage, and each of the metal windows has a substantially identical height with a depth of an inserting groove in which each such metal window is inserted and a depth of each of the metal windows is slightly wider than a width of an inserting groove in which each such metal window is inserted.
According to another preferred example of the filter, each of the inserting grooves has an identical depth whereas widths of the inserting grooves become gradually and symmetrically deeper to a middle stage, and each of the metal windows has a substantially identical height with a depth of an inserting groove in which each such metal window is inserted and a depth of each of the metal windows is slightly wider than a width of an inserting groove in which each such metal window is inserted.
Preferably, each of the metal windows is fixed as a single body on the parallel conductive plates.
Meanwhile, it is preferable that the filter assembly further comprises a plurality of tuning screws inserted, parallel to the parallel conductive plates toward the dielectric line, through both sidewalls of the filter housing, for tuning a resonance frequency of the filter by adjusting insertion lengths of the tuning screws.
The processing and assembling of the filter assembly is very simple according to the present invention. That is, the processing of major parts is completed once the inserting grooves are formed in the dielectric line made from a material which is difficult for processing, and once the metal windows respectively corresponding to the inserting grooves are arranged to form a straight line on inner surfaces of the parallel conductive plates. The assembling of the filter assembly is completed once the dielectric line is simply inserted in the parallel conductive plates of the filter housing to the effect that the metal windows are inserted in the corresponding inserting grooves.
Therefore, the filter assembly of the present invention has a simple structure and can remarkably reduce its manufacturing costs and maximize its productivity due to superior processing and assembling characteristics. Further, since the filter assembly of the present invention is designed to minimize the factors of error occurrence, the filter structure of the millimeter wave band which requires the precision of several microns can maintain the processing machine""s precision and the filter characteristic to a designed degree without having an extra auxiliary zig.
Other characteristics and advantages of the present invention will become more apparent with reference to the following detailed description of the invention and the attached drawings illustrating the characteristics of preferred embodiments.